Digital data processing systems typically include data storage devices, for example, multi-platter disk drives. Data are recorded on the disks in concentric tracks. The tracks are divided into sectors, and each sector is further divided into a header section and a data section. Data are recorded in (written) or retrieved from (read) the data portions of the sectors while address information, that is, disk surface number, track number and sector number, is stored in the header sections.
Data are read from or written to the disks using read/write heads. Each head is associated with a particular disk surface or portion of a disk surface. As the disks rotate under the heads, data are, for example, read from a particular sector when the associated head is over the data portion of that sector.
To begin a read or write operation the appropriate head is moved to the track containing the desired sector. The system then synchronizes various timers to the disk and precisely positions the head over the center of the track so that it can read the sector addresses rotating under the head, this is commonly referred to as format synchronization or synchronization to the disk. Once the system is synchronized to the disk, it locates the selected sector by reading the sector addresses and then it performs the read or write operation. If the system has used the head in the preceding read or write operation, or if it stays in synchronism with the disk using appropriate timers, for example during idle times, the system need not re-synchronize to the disk. Each time it uses a different head to perform a read or write operation, however, it must synchronize to the disk.
Using prior technology, most data processing systems record information for synchronization either on an extra servo surface or directly on the data surfaces. Such systems are referred to, respectively, as dedicated servo and embedded servo systems. Dedicated servo systems devote an entire disk surface to synchronization information, while embedded-servo systems devote a relatively small portion of each sector to synchronization information. Thus an embedded-only servo system typically records more data on a given number of disks than does a dedicated servo system.
An embedded-servo system must find the synchronization information before it can start synchronizing, and then it must fully synchronize to the disk in the time it takes the located synchronization information to rotate under the head. Otherwise, the system must wait for another sector containing embedded servo information to rotate under the head before it can complete its synchronization.
The speed with which the system can accurately synchronize to the disk affects the speed with which the system can transfer data to or from the drive. Accordingly, a system must balance the amount of disk space given to synchronization information against the amount of time it takes the system to synchronize to the disk.
A known embedded servo system records sector headers at radially aligned disk locations to enable the system to identify and interpret the header information quickly, even if the head is positioned slightly off of the center of a track. Once any header is located, the system can begin to synchronize to the disk using the synchronization information in the header. If the head can not completely synchronize to the recorded information in the time it takes the header to rotate under the head, the system waits a predetermined time for the next header and completes the synchronization process.
Sectors vary in physical length according to their locations along the radius of the disk. Thus, given the same angular extent for all the sectors, a sector located near the center of the disk is relatively short while a sector located at the outer edge of the disk is relatively long. The time it takes any sector to rotate under the head is a constant, however, because the disk rotates at a constant speed. The sectors thus each contain the same amount of data, regardless of their radial positions, assuming they are recorded at the same frequency.
A data signal is recorded on a disk as a series of magnetic flux reversals. The amount of data which can be recorded in a sector depends on how close together the flux reversals can be spaced without interfering with signal interpretation. The system interprets a flux reversal in a particular location in a sector as, for example, a binary ONE. It interprets the absence of a flux reversal in that particular location as a binary ZERO. If the flux reversals are so close together that the system can not determine if a flux reversal is or is not present at the particular location, the system can not accurately interpret the data. The amount of data recorded in any sector of a disk recorded at a single frequency is thus limited by the amount of data, or the density of the signal, which can be recorded in the shortest sector. The amount of data recorded in the shortest sector is commonly referred to as a "data block," and each sector, regardless of length, contains a single data block.
Several mechanisms have been developed to record more information on a disk. One such mechanism involves recording different portions of a disk at different densities, or signal frequencies. This is commonly referred to as "banding," with portions of the disk recorded at the same frequency being in the same "band." Known banded systems record information in the sectors closer to the outer edge of the disk, that is, the physically longer sectors, at a higher frequency than they record information in the sectors located closer to the center of the disk. This enables the system to record more than one data block in each of the longer sectors, and thus, more data on a given disk.
When information is recorded on a disk at different frequencies, one of the problems which must be solved is how to synchronize the system at the various frequencies. Each time a head is switched between bands, the system must resynchronize to the appropriate frequency before it can interpret the header addresses and find the desired sector.
Known banded systems synchronize to the various bands using a dedicated servo surface. However, as discussed, such dedicated servo systems sacrifice an entire disk surface for the synchronization information.